Gimp, software most useful in photography editing, it has strong resemblance of Photoshop, although it seems as the way filters are created show a different technique. For example 'Clothify' filter is a combination of other different filters to achieve a certainly interesting effect.

Inkscape is a vector program which offers many ways how to easily draw vector pictures, transform typography, turn 2D geometry to 3D, etc. Resembles Adobe Illustrator and CorelDraw, although has easier access to many more options.

Scribus is used for text, typography manipulation. A good software for digital publishing.

Tried Gaussian blur if it has any difference from Photoshop version.

This is quite interesting filter-'Clothify' its a combination of a few different filters which makes a new effect that resembles old cracked paint

'Clothify'

'Gaussian Blur'

GIMP

INKSCAPE

'Neon'

In comparison with Photoshop's version this filter seems more effective, the difference is clear.

This is Render>Clouds>Fog filter, I was surprised how natural it looks when applied to the right place.

Example of my libre graphics poster, using neon filter.

Scaled an image, found a part of the town houses that create interesting colour pallet and applied 'Mosaic' filter to make a pattern

'Clouds/Fog'

'Newsprint'

'Scale of the Universe'

With these snake pictures made by Mark Laita, his pictures have really high contrast so it fast comfortable to explore what filters work the best on bright colours.
In the last picture I decided to use Edge-Detect filters- 'Neon' and 'Sobel', because it works the best in this case, showing how simple yellow snake actually has more colour depth inside.

Another version of 'Scale of the Universe', simply scaled to a part of image that I was interested in the most, applied filters I learned about and experimented the most-'Neon', 'Edge',

'Scale of the Universe'

My exploration with different tools, playing around looking what could be used in what way. Everything is managed easy and there's many choices what to do. Although for 'Scale of the Universe' I choose simple drawing with 'Bezier' curves tool which has no boundaries when it comes to drawing whatever is on your mind.

'Snakes'